1. Field of the Invention
This invention relates to phosphate derivatives useful as fillers for human hard tissues having excellent mechanical strength and handling properties, a process for producing such phosphate derivatives and fillers for human hard tissues obtained from the phosphate derivatives. The term "human hard tissues" as used herein means human bones, teeth and the like. The fillers according to the present invention exhibit excellent properties for reparation of such hard tissues, particularly, they are used as fillers for dental treatment. The present invention as well as conventional techniques relating thereto are therefore described hereinafter mainly with reference to applications in the field of dentistry, but it is to be understood that these descriptions can also be applied to other human hard tissues and that the utility according to the present invention is not limited to the field of dentistry.
2. Description of the Prior Art
Hitherto, various filler materials for use in dentistry have been known and, of these conventional fillers, a so-called composite resin filler comprising polymerizable monomers and inorganic fillers has been widely used in reparation of front tooth because of its excellent mechanical strength, appearance, operability, etc. Also, such composite resin fillers are now used for reparation of temporary molars.
However, since molars are generally subjected to extremely high occlusal pressure, the fillers used for reparation of molars must have improved mechanical strength such as compression strength, bending strength and the like sufficient to endure such pressure and, therefore, conventional fillers could not be applied to molars having occlusal surfaces. One method has been known for improving the mechanical strength of composite resin fillers by using finely divided inorganic fillers and extensive studies have been made from this standpoint.
On the other hand, U.S. Pat. Nos. 3,066,112 and 3,179,623 disclose a glycidyl methacrylate derivative of bisphenol A (hereinafter abbreviated as "Bis-GMA") represented by the formula ##STR2## as a polymerizable monomer which provides an extremely hard resin and hence the above monomer has been mainly used as a composit resin filler. No polymeric monomers having properties superior to Bis-GMA have been developed thus far.
However, Bis-GMA has a defect in that it is very difficult to handle because of its highly viscous property unless it is diluted with a reactive diluent such as methyl methacrylate, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate and the like. The use of these reactive diluents may cause other problems, i.e., poor moisture resistance and decreased mechanical strength of the cured filler by absorbing a large amount of water. In addition, as the most basic problem, the above reactive diluents are strongly irritative to the affected part of patients since they have a relatively low molecular weight and thus are permeable to dental pulp tissues. It is therefore highly desirable to develop monomers which require only a small amount of these reactive diluents.
Also, in the conventional composit resin fillers, finely divided inorganic fillers are incorporated into polymerizable monomers and other components in order to improve the mechanical strength thereof, but it is also preferred to use polymerizable monomers having a viscosity lower than Bis-GMA and having a crosslinking property higher than Bis-GMA as a monomer component of the composite resin filler.
As a result of extensive studies, the present inventors previously proposed a compound which satisfies the above requirements, as disclosed in Japanese Patent Application Nos. 114995/80, 114996/80 and 133234/80, but it is still desirable to develop monomers having improved mechanical properties and operability. More specifically, the compounds disclosed in the above three Japanese patent applications are pyrophosphate derivatives having polymerizable double bonds such as those present in alkenyloxy, acryloyloxy(lower)alkyl, methacryloyloxy(lower)alkyl groups. These pyrophosphate derivatives exhibit satisfactory adhesive properties for bones and teeth, but appear to require further improvements as a filler for reparatory use.